A battery having high performance and large capacity battery has been in increasing need for the use as a power source for a portable electronic device, an electric vehicle, and the like.
The battery generates electric power by using materials capable of electrochemical reactions in the positive electrode (cathode) and negative electrode (anode). Representative examples of the high performance battery may include a lithium secondary battery which generates electrical energy based on a change in chemical potential when lithium ions are intercalated or deintercalated in the cathode and anode.
The lithium secondary battery may provide high energy density by using organic electrolytic solutions and have a discharge voltage two or greater than that of a battery in which the mere alkaline aqueous solution is used. The lithium secondary battery may be manufactured by using materials capable of converting reversible intercalation/deintercalation of lithium ions as cathode and anode active materials, and filling an organic electrolyte solution or polymer electrolyte solution between the cathode and the anode.
Although research and development on the batteries have been continued for about 20 years, the lithium secondary battery may have a limitation in energy capacity since a combination of oxide or a phosphate material in the cathode and graphite in the anode have been used.
Therefore, in order to apply the lithium secondary battery, particularly, to an application for an electric vehicle, an electrode material having high Li storage capability may be required for both cathode and anode. Silicon (Si) may have the highest Li-alloying capability, for instance, of about 3,800 mAh/g, which is about ten times greater than Li-alloying capability of graphite. Accordingly, Si may be the most suitable anode material. However, a large volume change may occur in Si which is a host material when Li is alloyed with Si, and may cause cracking and rapid pulverization of a Si-based anode. In this regard, significant enhancement in the service life and charge-discharge rate of the anode has been obtained over several decades by introducing a Si nanostructure, particularly, nanowires. Due to a high surface-to-volume ratio of the Si nanostructure, the nanostructure may be more resistant to stress caused by the surface effect thereof than other structures, and may be strong against cracking.
In related arts, methods of synthesizing Si nanowires for an application for Li-based battery anodes have been provided; for example, a growth method and an etching method.
An example of the growth method which may be performed in a chemical vapor deposition (CVD) reactor may be a very-large-scale technology. Meanwhile, the etching method may obtain Si nanowires by performing etching from bulk crystalline Si, for example, Si wafer and Si powder.
In the growth method, Si nanowires may be grown directly on a current collector of an anode. When etched Si nanowires are used, the Si nanowires may be included in a binder, for instance, polymer binder, which may contain slurry and a conductive additive, such as carbon-based powder. The slurry may be generally deposited on the current collector of a battery electrode by a tape casting and dried in an oven prior to use, thereby removing a solvent from the slurry.
However, some technical problems during growth of Si nanowires have been reported in the related art.
For example, an active material may be diluted by using a binder and a conductive powder, indicating only a part of a given anode mass may contribute to Li storage. Similar problem may also occur in a graphite-based anode. In comparison to the theoretical capacity of graphite of about 372 mAh/g, the commercial anode manufactured by the slurry process may provide only up to 275 to 300 mAh/g.
In addition, when Si nanowire is expanded by alloying with Li, the Si nanowires may “push back” a surrounding binder and conductive particles. In particular, when the Si nanowires are contracted and Li as an electrolyte is released during the use of a battery, the surrounding binder and particles may dislocate from the original positions. Accordingly, some nanowires may be disconnected from a current collector, thereby causing a capacity loss of the electrode.
Furthermore, the thickness, or a charge capacity per cm2 of the anode, of an anode material on the current collector may be limited by delamination that rapidly occurs when the thickness is in a range of several tens of microns.
In the related arts, a method of manufacturing nickel silicide which includes: (a) forming a ruthenium layer on a substrate; (b) forming a nickel layer on the ruthenium layer by CVD; and (c) forming nickel silicide by subjecting the ruthenium layer and the nickel layer to heat treatment at 300 to 1,100° C. for 40 seconds has been provided.
In addition, a technology of manufacturing nickel silicide of nonvolatile nanocrystal (NC) embedded in oxide and nitride layers by removing oxides and microparticles on a p-type silicon wafer by an RCA method, growing a 3-nm tunnel oxide in an atmospheric CVD system by a dry oxidation process to deposit an Ni0.3Si0.7 layer, and performing annealing under a nitrogen atmosphere by a rapid thermal annealing (RTA) process has been introduced. In another example, a technology of manufacturing pure NiSi nanowires including a polycrystalline NiSi2 core having a thickness of about 30 nm, SiO shell nanowires doped with amorphous Ni, and nickel on a Ni catalyst layer deposited in a thickness of about 2 nm on a stainless steel substrate by a CVD furnace using silane has been developed. In addition, a technology of manufacturing an ordered Si nanowire with NiSi2 tip arrays by reacting a nickel thin film on an ordered silicon nanowire array coated with silica has been provided.
However, above-described technologies in the related arts still do not provide further solution, such as deterioration of physical properties or structural defects caused by the capacity loss or thickness limitation.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.